feat: Add public RSS feed for conferences

Users currently have no way to be notified when new conferences are
published on an OSEM instance. This adds an RSS 2.0 feed endpoint at
/feed.rss that lists all conferences ordered by creation date.

The feed action is excluded from CanCanCan authorization so it remains
publicly accessible without authentication, similar to the existing
calendar endpoint.

Also adds an RSS subscribe link to the conferences index page and an
auto-discovery <link> tag in the application layout so RSS readers can
detect the feed automatically.

Fixes #3543

app/controllers/conferences_controller.rb

@@ -3,7 +3,8 @@
 class ConferencesController < ApplicationController
   protect_from_forgery with: :null_session
   before_action :respond_to_options
-  load_and_authorize_resource find_by: :short_title, except: :show
+  load_and_authorize_resource find_by: :short_title, except: [:show, :feed]
+  skip_authorization_check only: :feed
 
   def index
     @current    = Conference.upcoming.reorder(start_date: :asc)
@@ -108,6 +109,13 @@ class ConferencesController < ApplicationController
     end
   end
 
+  def feed
+    @conferences = Conference.order(created_at: :desc)
+    respond_to do |format|
+      format.rss { render layout: false }
+    end
+  end
+
   def code_of_conduct; end
 
   private

app/views/conferences/feed.rss.builder

@@ -0,0 +1,18 @@
+xml.instruct! :xml, version: "1.0"
+xml.rss version: "2.0" do
+  xml.channel do
+    xml.title "#{ENV.fetch('OSEM_NAME', 'OSEM')} - Conferences"
+    xml.description "Conferences from #{ENV.fetch('OSEM_NAME', 'OSEM')}"
+    xml.link conferences_url
+
+    @conferences.each do |conference|
+      xml.item do
+        xml.title conference.title
+        xml.description conference.description
+        xml.pubDate conference.created_at.to_fs(:rfc822)
+        xml.link conference_url(conference.short_title)
+        xml.guid conference_url(conference.short_title)
+      end
+    end
+  end
+end

app/views/conferences/index.html.haml

@@ -23,6 +23,9 @@
     %span.btn-group
       = link_to("Days only", calendar_url(protocol: 'webcal', format: 'ics'), class: 'btn btn-default')
       = link_to("Detailed", calendar_url(protocol: 'webcal', format: 'ics', full: true), class: 'btn btn-default')
+  %p
+    Subscribe to the RSS feed:
+    = link_to("RSS Feed", feed_url(format: :rss), class: 'btn btn-default')
 
 -content_for :script_body do
   :javascript

app/views/layouts/application.html.haml

@@ -7,6 +7,7 @@
     %meta{content: '', name: 'author'}
     = stylesheet_link_tag "application", media: 'all'
     = javascript_include_tag "application"
+    = auto_discovery_link_tag(:rss, feed_url(format: :rss), title: "#{ENV.fetch('OSEM_NAME', 'OSEM')} Conferences RSS Feed")
 
     = csrf_meta_tags
     = content_for(:script_head)

config/routes.rb

@@ -216,6 +216,7 @@ Osem::Application.routes.draw do
   get '/admin' => redirect('/admin/conferences')
 
   get '/calendar' => 'conferences#calendar'
+  get '/feed' => 'conferences#feed'
 
   if ENV.fetch('OSEM_ROOT_CONFERENCE', nil)
     root to: redirect("/conferences/#{ENV.fetch('OSEM_ROOT_CONFERENCE')}")